In the combustion of a fuel, such as coal, oil, peat, waste, etc, in a combustion plant, such as a power plant, a hot process gas is generated. Such a hot process gas, often referred to as a flue gas, contains, among other components, carbon dioxide (CO2). The negative environmental effects of releasing carbon dioxide to the atmosphere have been widely recognized, and have resulted in the development of processes adapted for removing carbon dioxide from the hot process gas generated in the combustion of the above mentioned fuels.
WO 2006/022885 relates to a process for absorbing carbon dioxide from a flue gas. The process comprises treatment of the flue gas by means of conventional air pollution control processes, such as by means of particulate collectors, NOx and SO2 control, acid mist capturing device etc. After the conventional air pollution control processes, the flue gas has a temperature of about 40-70° C. Subsequently, the flue gas is cooled down to, preferably, 0-25° C. by means of direct contact cooling, in which the flue gas is cooled by means of cold water. Following cooling, the flue gas is brought to a CO2 absorber, in which the flue gas is brought into contact with a low temperature ammoniated slurry or solution having a low carbon dioxide content. The carbon dioxide of the flue gas is absorbed into the ammoniated slurry or solution, and a clean flue gas, containing very small amounts of pollutants and carbon dioxide, leaves the CO2 absorber. The carbon dioxide rich ammoniated slurry or solution is regenerated in a regenerator, in which the carbon dioxide is stripped, at a temperature of about 50-200° C. and under high pressure, to form a concentrated carbon dioxide rich stream.
A problem of prior art processes such as the one described in WO 2006/022885 is that the clean flue gas released from the CO2 absorber will contain a substantial amount of ammonia. WO 2006/022885 for example suggests a cold water wash of the flue gas before releasing it to the atmosphere. Such water wash can reduce the ammonia concentration in the clean flue gas to the range of about 100-1000 ppm, or even to the range of about 100-400 ppm, but this is still too high for discharging such a flue gas to the atmosphere. In addition, the capital costs for setting up such a process, including the appropriate equipment for water wash, are very high.